rbx:debug> ?
Available commands:
  b(reak)                   List breakpoints
  b(reak)           Set a breakpoint at the start of the specified method
  c(ont)                    Continue execution to next breakpoint, or end of program (whichever comes first)
  d(ecode) [start [end]]    Decode bytecode around breakpoint or between start/end
  h(elp)                    Display this list of commands
  l(ist)   [start [end]]    List source code lines around breakpoint or between start/end
  n(ext) i(nstruction) [n]  Step to the next (or nth next) VM instruction without stepping into called methods
  o(ut)                     Return to caller and then break
  q(uit)                    Remove all breakpoints and resume program
  v(ars)                    Show local variables and their values
  v(m) s(tack)              Display the current objects on the VM stack
  w(here)                   Show execution backtrace
                            Any other command is assumed to be a Ruby expression, and is evaluated.